The present invention relates generally to integrated circuits and, more particularly, to a rail-to-rail comparator circuit.
Comparators are a common type of circuit used in many applications, some of which require that the comparator operate over a wide common-mode input voltage range. As improvements in technology have led to lower device supply voltages and smaller threshold voltage ratios, integrated circuit designers have found it increasingly more difficult to design and manufacture comparators that operate over a wide common-mode input voltage range. Typically, as the common-mode voltage at the input of the comparator approaches either the positive supply rail or the negative supply rail of the device, the comparator ceases to function properly, resulting in an output signal that is not indicative of the signals at the input of the comparator.
One known comparator design, discussed in U.S. Pat. No. 7,576,572 implements a rail-to-rail comparator by paralleling two complementary, symmetrical, operational transconductance amplifiers (OTAs). The design, however, has redundant components and uses more active loads than necessary. Furthermore, the parasitic capacitances introduced by the redundant loads slow down the comparator.
Another known comparator design, discussed in “A rail-to-rail input-range CMOS voltage comparator,” 1997 Proceedings of the 40th Midwest Symposium on Circuits and Systems, Vol. 1, implements a rail-to-rail comparator using two symmetrical OTAs with a cross-coupled load. In this design, the cross-coupled load can increase the DC gain of the circuit resulting in poor common-mode voltage rejection. Further, propagation delay of the design is increased if the internal node is clamped to a power or ground rail. Additionally, this circuit requires switches to overcome the circuit's inability to automatically manage both high and low common-mode voltages. Switches add more parasitic capacitance.
It would be desirable to provide a comparator that is operational over a full rail-to-rail common-mode input voltage range, satisfies high-speed operational requirements, achieves low power consumption, and places few demands on the manufacturing process.